1. Field of the Invention
The present invention relates generally to the field of computer graphics and pertains more particularly to an apparatus for and a method of enhancing shape perception in a computer graphics system.
2. Discussion of the Prior Art
The perception of shape and surface detail is crucial for the interpretation of images in a wide variety of disciplines. Examples include such diverse fields as the study of ancient artifacts and remains, archeology, forensics, diagnostic medicine, industrial inspection, and manufacturing control. The field of image enhancement provides a variety of conventional methods that can assist in making surface detail more apparent. For example, the scanner signal for electronically scanned images can be passed through an electrical filter with a high-frequency bandpass characteristic. More typically, two-dimensional (2D) image operators are conventionally used for enhancing detail in images of samples. There are many such operators currently available. One common method is to perform a convolution of the image with a 2D function that provides enhancement of high frequency content relative to low-frequency content. This is one method of implementing a 2D high pass filter.
Another possibility for scanned images is the technique of unsharp masking. In this process, the image is effectively scanned by two overlapping apertures, one at normal resolution and the other at a lower resolution, producing normal and low-resolution images, respectively. A masked image is then formed electronically with a proportionality constant. Typically the constant is such that the ratio of normal to low-resolution components in the masked image is from 1.5:1 to 5:1. Subjectively, the apparent sharpness of the image is improved.
One form of edge enhancement called statistical differencing involves the generation of an image by dividing each pixel value by its measured statistical standard deviation. The standard deviation is computed over some neighborhood of the pixel coordinate. The mean value of the original image at the pixel coordinate is approximated by blurring or smoothing the original image by a smoothing operator of low-pass form. This mean value is divided by the standard deviation to result in the enhanced image value. The enhanced image will be increased in amplitude with respect to the original image at edge points that deviate from their neighbors and decrease in relative amplitude elsewhere. Unfortunately, noise components may also be accentuated by this technique.
One of ordinary skill in the art will be aware of any number of other conventional enhancement techniques. Contrast enhancement methods in particular, such as high pass filtering, unsharp masking, and statistical differencing can be somewhat useful for making surface detail more apparent. However, these operations are inherently 2D and are therefore limited. In contrast, the present invention draws upon three-dimensional (3D) information extracted from multiple 2D images simultaneously and therefore provides a fundamentally new and different capability.
A definite need exists for a system having an ability to use 3D information to enhance the perception of surface detail and shape. In particular, a need exists for a system which is capable of allowing the user to modify reflectance properties and lighting conditions in a skillful manner. Ideally, such a system would have a lower cost and a higher productivity than conventional systems. With a system of this type, system performance can be enhanced. A primary purpose of the present invention is to solve this need and provide further, related advantages.
A graphics system is disclosed that enhances shape perception. The presently preferred embodiment of the system utilizes the mathematical machinery of parametric texture maps. In general, parametric texture maps allow the graphics system to render surface features of a 3D object in a manner that is direction dependent but without the time consuming and expensive calculations involved in the evaluation of lighting equations on a per pixel basis. A parametric texture map holds a set of parameters that define a surface structure in a manner in which the appearance of the surface structure varies in response to a direction vector. The direction vector may be any user-defined vector including a light source vector or a half-angle vector. The parameters are those of a predetermined equation, the evaluation of which does not involve vector calculations. The equation may take any form including a polynomial equation or a non-polynomial equation. The graphic system renders a polygon with the surface structure using the equation.